BungeeCord is a very useful software created by the SpigotMC Team in 2013 and made to connect multiple minecraft servers between them, it acts as a proxy between the player's client and the servers, you can navigate between servers using commands or portals, i t is made to separate servers between gamemodes like creative or survival or to optimize the load of players on the server, in order to work, it requires a minimum of 3 servers, 2 to be connected between them and 1 to run the BungeeCord Software, its direct rival is Waterfall.